docs: add verification guide (#1885)

Author: yury-s

docs/README.md

@@ -31,9 +31,12 @@
     - [Handle requests](./network.md#handle-requests)
     - [Modify requests](./network.md#modify-requests)
     - [Abort requests](./network.md#abort-requests)
-1. Scraping and verification
-    - Screenshots
-    - Evaluation
+1. [Scraping and verification](./verification.md)
+    - [Screenshots](./verification.md#capturing-screenshot)
+    - [Evaluation](./verification.md#evaluating-javascript)
+    - [Console messages](./verification.md#listening-console-messages)
+    - [Uncaghut exceptions](./verification.md#uncaghut-exceptions)
+    - [Page crashes](./verification.md#page-crashes)
 1. [Navigation and loading](./loading.md)
     - [Common scenarios](./loading.md#common-scenarios)
     - [Loading a popup](./loading.md#loading-a-popup)

docs/verification.md

@@ -0,0 +1,164 @@
+# Scraping and verification
+
+Playwright allows verifiying state of the page and catching abnormal behavior by:
+  - evaluating JavaScript code snippets in the page
+  - capturing screenshots (png or jpeg)
+  - listening console messages
+  - observing uncaghut exceptions in the page
+  - observing page crashes
+  - etc
+
+#### Contents
+- [Evaluating JavaScript](#evaluating-javascript)
+- [Capturing screenshot](#capturing-screenshot)
+- [Listening console messages](#listening-console-messages)
+- [Uncaghut exceptions](#uncaghut-exceptions)
+- [Page crashes](#page-crashes)
+
+<br/>
+
+## Evaluating JavaScript
+
+Execute JavaScript function in the page:
+```js
+  const href = await page.evaluate(() => document.location.href);
+```
+
+If the result is a Promise or if the function is asynchronouse eveluate will automatically wait until it's resolved:
+```js
+  const status = await page.evaluate(async () => {
+    const response = await fetch(location.href);
+    return response.status;
+  });
+```
+
+Get object handle and use it in multiple evaluations:
+```js
+  // Create a new array in the page, wriate a reference to it in
+  // window.myArray and get a handle to it.
+  const myArrayHandle = await page.evaluateHandle(() => {
+    window.myArray = [1];
+    return myArray;
+  });
+
+  // Get current length of the array using the handle.
+  const length = await page.evaluate(
+    (arg) => arg.myArray.length,
+    { myArray: myArrayHandle }
+  );
+
+  // Add one more element to the array using the handle
+  await page.evaluate((arg) => arg.myArray.push(arg.newElement), {
+    myArray:myArrayHandle,
+    newElement: 2
+  });
+
+  // Get current length of the array using window.myArray reference.
+  const newLength = await page.evaluate(() => window.myArray.length);
+
+  // Release the object when it's no longer needed.
+  await myArrayHandle.dispose();
+```
+
+#### API reference
+
+- [page.$(selector)](./api.md#pageselector)
+- [page.$$(selector)](./api.md#pageselector-1)
+- [page.$eval(selector, pageFunction[, arg])](./api.md#pageevalselector-pagefunction-arg)
+- [page.$$eval(selector, pageFunction[, arg])](./api.md#pageevalselector-pagefunction-arg-1)
+- [page.evaluate(pageFunction[, arg])](./api.md#pageevaluatepagefunction-arg)
+- [page.evaluateHandle(pageFunction[, arg])](./api.md#pageevaluatehandlepagefunction-arg)
+
+<br/>
+
+## Capturing screenshot
+
+Take screenshot of the page's viewport and save it in a png file:
+```js
+  await page.screenshot({path: 'screenshot.png'});
+```
+Capture entire scrollable area of the page:
+```js
+  await page.screenshot({path: 'screenshot.png', fullPage: true});
+```
+Save screenshot in an in-memory buffer (the content is base64-encoded image bytes):
+```js
+  const buffer = await page.screenshot();
+  // Log the length.
+  console.log(buffer.length);
+```
+
+
+#### API reference
+
+- [page.screenshot([options])](./api.md#pagescreenshotoptions)
+- [Node.js Buffer](https://nodejs.org/api/buffer.html)
+
+<br/>
+
+## Listening console messages
+
+Listen all console messages in a page and dump _errors_ in to the terminal:
+
+```js
+  // Get all console messages from the page.
+  page.on('console', msg => {
+    // Handle only errors.
+    if (msg.type() !== 'error')
+      return;
+    console.log(`text: "${msg.text()}"`);
+  });
+
+  await page.evaluate(() => console.error('Page error message'));
+```
+Get access to the console message arguments:
+```js
+  page.on('console', msg => {
+    for (let i = 0; i < msg.args().length; ++i)
+      console.log(`${i}: ${msg.args()[i]}`);
+  });
+```
+
+#### API reference
+
+- [class: ConsoleMessage](./api.md#class-consolemessage)
+- [class `JSHandle`](./api.md#class-jshandle)
+- [event: 'console'](./api.md#event-console)
+
+<br/>
+
+
+## Uncaghut exceptions
+
+Listen uncaghut exceptions in the page:
+```js
+  // Log all uncaught errors to the terminal
+  page.on('pageerror', exception => {
+    console.log(`Uncaught exception: "${exception}"`);
+  });
+
+  // Navigate to a page with an exception.
+  await page.goto('data:text/html,<script>throw new Error("Test")</script>');
+```
+
+#### API reference
+
+- [event: 'pageerror'](./api.md#event-pageerror)
+
+<br/>
+
+## Page crashes
+
+Listen to page crashes:
+```js
+  page.on('crash', exception => {
+    console.log(`Page crashed`);
+  });
+```
+It's very unusual for page to crash but might happen if a page allocates too much memory or due to a bug in a browser.
+
+#### API reference
+
+- [event: 'crash'](./api.md#event-crash)
+
+<br/>